Cochlear implants are implantable systems, which can provide hearing to profoundly deaf or severely hearing impaired persons. Unlike conventional hearing aids that mechanically apply an amplified sound signal to the middle ear, a cochlear implant provides direct electrical stimulation to multiple implanted electrodes that excite the acoustic nerve in the inner ear, thus giving a sensation of hearing.
While cochlear implants improve the quality of life for many people worldwide, electric hearing still lacks the resolving ability found in normal hearing. Electrical hearing is not equivalent to normal hearing because the activation of the auditory nerve by cochlear implant electrode currents is not faithful to the pattern produced by normal physiological mechanisms. Notably, electric stimulation lacks the spatial and temporal resolution of normal hearing mechanism, reasons that are known to degrade ability of CI users to understand speech in noisy environment and to localize sound.
Most current cochlear implants use speech coding strategies that encode acoustic signals into electrical pulses for stimulating the acoustic nerve. One common speech coding strategy is the so called “continuous-interleaved-sampling strategy” (CIS). The CIS speech coding strategy samples the signal envelope within individual frequency bands at predetermined time intervals, providing an increased level of speech understanding merely by coding the amplitude modulation (AM) of the speech signal in each band. Each electrode is stimulated with an intensity corresponding to the instantaneous amplitude of the corresponding filter channel. In this strategy, the analysis period is predetermined and hence the frequency of stimulation for each electrode is more or less fixed.
Another coding strategy—Fine Structure Processing (FSP) analyzes the phase of the band pass signals and synchronizes the stimulation pulses with specific events in the phase of the corresponding electrode. In FSP coding, time events are defined using the zero crossings of the band pass signal where all channels are stimulated sequentially in a predetermined order. Other approaches providing some temporal fine structure information include—Peak Derived Timing (PDT) coding that derives the timing of stimulation pulses from the positive peaks in the band pass signals. Spike-based Temporal Auditory Representation (STAR) strategy extracted the pulse timing from the zero crossings of the band pass signals.
It is accepted that listening with two ears rather than one, for normal listeners, allows improved speech intelligibility in noise as well as the ability to better determine sound direction. Studies with both normal hearing and hearing impaired listeners has shown significant enhancement in performance during diochotic (different sounds in two ears) listening when compared to diotic (same sounds in two ears) listening. The improvement can be measured as a binaural intelligibility level difference (BILD), defining the difference in signal level between two binaural conditions for a given percent intelligibility. The enhancement is known to be a function of both interaural level differences cues (ILDs) and interaural time difference cues (ITDs). Similarly, localisation in the horizontal plane has been shown to be a function dependent on these ILD and ITD cues.
The disclosure offers an alternative to current coding techniques—a stimulation strategy that may better mimic the neural firing patterns of a healthy cochlea in order to present stimulation pulses at more physiologically proper times and locations in the cochlear implant. Thus, a method and system of processing sound waves into electrical signals that capture Fine Time Structure (FTS) information in incoming sounds and accurately direct this information to appropriate nerves that spatially innervate the cochlea is presented.